1. Field of the Invention
The invention relates to the production of microporous polymeric films for numerous uses ranging from high energy physics targets, biomedical structures for tissue ingrowth, filters, low dielectric films for electronic devices, asymmetric membranes, and the like. In particular, the microporous polymer films of the invention are produced by exposing a pressurized or dense gas (may be liquified or supercritical) that is not a solvent for the polymer or wherein the polymer is only sparingly soluble, to a polymer film that contains substantial solvent which is soluble in the dense gas. This results in the phase separation of the polymer/solvent film into two phases: a solvent-rich phase and a polymer-rich phase. Crystallization or glassing of the polymer, followed by removal of the solvent-rich phase and the dense gas then produces the dry microporous polymer films of the invention.
2. Description of the Related Art
Microporous polymeric films have numerous uses including: high-energy physics targets, biomedical devices, drug delivery systems, filters, asymmetric membranes, low dielectric films for electronic devices, and the like. To date, such microporous polymeric films have been produced by a variety of techniques, each of which suffers various disadvantages. In particular, certain methods require the use of large quantities of solvents, which are either toxic, or pose health, environmental, or safety concerns. In general, to make a microporous polymer film, a polymer/solvent film is exposed to a non-solvent bath which is usually an organic liquid in which the polymer is not soluble but in which the solvent from the film is soluble. After phase separation in the film, the solvent in the film is extracted by the non-solvent in the bath. Residual solvent and non-solvent in the film is later evaporated leaving behind a microporous polymer film. This process results in a requirement for a large non-solvent bath and solvent emissions into the air, either of which may pose environmental, disposal, and health concerns. Depending upon the desired end use of the foam, it is necessary to select a polymer that possesses suitable characteristics, e.g., for use in biomedical devices, the polymer should be biocompatible, and, in some instances, biodegradable.
In the case of microporous polymer films, several parameters are important. These parameters include the thickness of the film, the density of the film, the size and nature of the cells of the foam (whether the foam is open or closed celled), the adherence or adheribility of the foam to various substrates, and the presence and nature of any dense surface on the film. Once a polymer has been selected, these parameters may be influenced by selecting the solvent, the proportion of solvent to polymer, and the process used for producing the microporous polymer film.
There are several processes for making non-porous polymer/solvent films, including spin coating, dip coating, and doctor-blading, among others. In the spin coating technique, a polymer solution of predetermined concentration is poured onto the turntable which is then rotated at a predetermined speed. The speed of rotation, concentration of polymer solution (and hence viscosity of the polymer solution) determines the thickness of film that forms on the turntable. In the dip coating technique, an object to be coated is dipped into a solution of the desired polymer and then withdrawn. The thickness of the film is largely dependent upon the concentration (and hence viscosity) of the dipping solution and the speed of removal of the object being coated from the dipping solution. In the doctor-blading technique, a thin film of a polymer solution is provided on a substrate surface and a sharp blade (a doctor blade) traverses the surface to remove excess polymer solution and produce a thin film of the desired thickness.
As mentioned before, current techniques for producing a thin microporous polymer film requires subjecting a polymer/solvent film to quantities of solvents which may be objectionable from a health, environmental, or safety standpoint. There yet exists a need for a more universally applicable process that permits the use of a wide range of polymers with a minimal quantity of organic solvents while producing microporous polymeric film of a desired thickness, cell size, and cell morphology.